WO2009141628A1

WO2009141628A1 - Method of testing for antiphospholipid antibodies

Info

Publication number: WO2009141628A1
Application number: PCT/GB2009/001298
Authority: WO
Inventors: Beverley J. Hunt
Original assignee: Guy's And St. Thomas's Nhs Foundation Trust
Priority date: 2008-05-23
Filing date: 2009-05-22
Publication date: 2009-11-26
Also published as: GB0809511D0

Abstract

The present invention provides methods of testing for an antiphospholipid antibody in a patient, comprising adding heparin to a blood sample from said patient and determining the activity or level of heparin in said blood sample.

Description

METHOD OF TESTING FOR ANTIPHO SPHOLIPID ANTIBODIES

The present invention relates to testing for antiphospholipid antibodies. In particular, the invention relates to a blood test for antiphospholipid antibodies that uses heparin.

The antiphospholipid syndrome is the association of antiphospholipid antibodies with arterial and/or venous thromboembolism and/or pregnancy morbidity (Miyakis et ah, Journal ofTJirombosis and Haemostasis 2006; 4: 295-306).

Antiphospholipid syndrome is currently detected either by coagulation assays to detect the lupus anticoagulant (LA) or ELISA assays to detect anticardiolipin and beta-2-glycoprotein antibodies, present on at least two occasions more than twelve weeks apart (Miyakis et ah, supra).

The LA was first described by Conley and Hartmann at The Johns Hopkins University in 1952. The LA prolongs phospholipid dependant clotting assays but paradoxically is procoagulant in vivo. It belongs to a heterogeneous family of autoimmune antibodies that include anticardiolipin antibodies that are directed at negatively charged phospholipid. The LA is the antiphospholipid antibody that has been most closely associated with a thrombotic tendency.

Dilute Russell's Viper Venom time (dRVVT) is one of the main tests currently used to detect the LA.

The dRWT is based on the ability of the venom of the Russell's viper to induce thrombosis and is described, for example, in Thiagarajan et a {Blood, 68(4): 869- 874, 1986). The coagulant in the venom directly activates factor X, which converts prothrombin into thrombin in the presence of factor V and phospholipid. Appropriate concentrations of Russell's viper venom and phospholipid are used to give a defined clotting time and make the test sensitive to LA. The inhibitory effect of LA on the dRWT can be overcome by adding an excess of phospholipid to the assay. The ratio of clotting time in the presence and absence of phospholipid is therefore used to determine the presence of LA.

Heparin is a highly-sulfated glycosaminoglycan and is widely used as an injectable anticoagulant. Currently, many patients with previous thrombosis and antiphospholipid molecules require long-term oral coagulation. Both unfractionated heparin (UFH) and low molecular weight heparins (LMWH) are used widely in these patients. Heparins are used to cover immediate treatment of an acute thrombotic event, to manage women during pregnancy with previous thrombosis and/or foetal loss, or during interventional cardiovascular procedures.

The currently available tests for antiphospholipid antibodies are difficult and time- consuming to carry out and poor to standardise. There is therefore a need in the art for a new and improved test for antiphospholipid antibodies.

The present inventor has found that when heparin is added to LA blood, it produces less measured anticoagulant activity compared to blood from healthy controls.

Accordingly, the present invention provides a method of testing for an antiphospholipid antibody in a patient, comprising adding heparin to a blood sample from said patient and determining the activity of heparin in said blood sample.

The present invention provides new tests for antiphospholipid antibodies that are easy and quick to carry out and conveniently use commonly available reagents.

Methods of the present invention are suitably carried out in vitro, for example under ex vivo conditions.

The methods of the invention involve the step of adding heparin to a blood sample from a patient. This step can involve adding heparin to a vessel containing the blood sample or adding a blood sample to a vessel containing the heparin. The heparin used in the present invention is typically unfractionated heparin (UFH) or a low molecular weight heparin (LMWH). Low molecular weight heparins include, for example, enoxaparin, tinzaparin, dalteparin, certoparin, parnaparin, reviparin, bemiparin and nadroparin, and also fondaparinux, a synthetic anti-Xa agent. In some embodiments of the invention, for example when the activated clotting time (ACT) or the activated partial thromboplastin time (aPTT) is used to determine heparin activity, the heparin is typically UFH.

Heparin can be used in the methods of the invention at any suitable concentration. Typically, heparin is diluted before use to give the desired final concentration. Heparin can be diluted with any suitable diluent, for example distilled water, deionised water, saline or dextrose.

Unfractionated heparin is typically used in the methods of the invention at a concentration of 1 international unit/millilitre (iu/ml) to 5 iu/ml, for example 1 iu/ml, and more typically at a concentration of 3 iu/ml to 5 iu/ml, for example 3 iu/ml, 4 iu/ml or 5 iu/ml. Such heparin concentrations are similar to those achieved during full heparinisation for cardiopulmonary bypass.

The blood sample can be a sample of whole blood or a sample of plasma or serum. In some embodiments of the invention, for example when the ACT is used to determine heparin activity, the blood sample is typically a sample of whole blood.

The methods of the invention can involve the step of obtaining the blood from a patient. The blood sample can be obtained from a patient by any suitable means, for example using flawless venepuncture.

The patient is typically human, but the invention is suitable for use in both human or veterinary medicine.

The method of the invention comprises determining the activity of heparin in the blood sample. Heparin is an anticoagulant and so determining the activity of heparin in the blood sample typically involves measuring the anticoagulant activity of heparin using a coagulation assay.

The activity, typically the anticoagulant activity, of heparin in the blood sample can be measured using any suitable means. One example of a test for measuring the activity of heparin is the activated partial thromboplastin time (aPTT). Accordingly, in one embodiment of the invention, the activity of heparin in the blood sample is determined by measuring the aPTT of the blood sample. The aPTT is typically carried out on plasma.

The aPTT is used to evaluate the intrinsic clotting system and is described, for example, in Struver et al. (Surg. Forum 13: 127-129, 1962). The test involves saturating a sample from a patient with a particulate activator such as celite or kaolin, thereby ensuring that all available Factor XII is converted to Factor XIIa. The test thus uses the intrinsic pathway of coagulation by activating the contact activation system, thereby initiating an enzymatic cascade from one factor to another. The reaction is then timed until the first signs of a clot appear. The aPTT thus involves adding a sample of blood from a patient to a contact activator and determining the length of time taken to form a blood clot. The aPTT is typically measured in seconds (sees).

Changes in the aPTT are proportional to the concentration of heparin, and the degree of prolongation of the aPTT normally gives an indication of a patient's level of anticoagulation.

The ACT is a similar test to the aPTT but is performed on whole blood. The ACT was originally described by Hattersley PG (JAMA; 196(5):436-40, 1966) and is used to monitor heparin levels during cardiopulmonary bypass, aiming to exceed an ACT time of greater than 480 seconds, which is normally achieved by a 300-400 iu/kg loading dose of unfractionated heparin. During bypass, the ACT is repeated regularly and if the ACT necessitates, additional heparin is administered. The ACT has been in use as a nearside monitoring tool for heparin since the mid- 1970 's when it was introduced to guide safe anticoagulation for cardiopulmonary bypass. Automated ACT machines were introduced and led to a reduction in blood loss. Today, the routine use of the ACT in monitoring adequacy of high-dose heparin anticoagulation has extended to other clinical areas aside from cardiac surgery to interventional cardiology, vascular surgery and other fields where heparin infusions are used.

In the present invention, the ACT and aPTT can be measured by any suitable method known in the art, typically using commercially available products and reagents. For example, the ACT can be measured using an "Actalyke" ACT tube (manufacturer Hemochron; activators: kaolin, celite and glass beads) and an Actalyke™ analyser (Helena Laboratories; distributed by GTA UK Ltd, Nottingham). Automated test systems such as Actalyke provide pre-loaded, disposable test tubes to which a blood sample is added. The test tube is then inserted into the instrument where the tube is rotated and warmed to 37⁰C until a fibrin clot is mechanically detected. When a clot is detected, the test terminates, a buzzer is sounded and the ACT is displayed (in seconds) on the instrument LED.

The present invention provides methods of testing for an antiphospholipid antibody. The invention thus encompasses methods of testing for one or more antiphospholipid antibodies. The antiphospholipid antibodies can be a combination of antiphospholipid antibodies. In one embodiment, the invention is used to test individuals with one or more medical symptoms of antiphospholipid antibodies, such as arterial and/or venous thromboembolism and/or pregnancy morbidity and/or cardiac abnormalities. In other embodiments, the invention is used to test individuals who do not currently have medical symptoms of antiphospholipid antibodies, for example as part of a thrombophilia screen.

The present invention encompasses methods of testing for the LA. As stated above, the LA is the antiphospholipid antibody that has been most closely associated with a thrombotic tendency and thus antiphospholipid syndrome. Thus, in one embodiment the antiphospholipid antibody is the LA.

The present invention relates to methods of testing for antiphospholipid antibodies and thus also relates to methods of testing for antiphospholipid syndrome.

The present invention may also be used to test for antithrombin deficiency (also referred to as antithrombin III deficiency). Antithrombin deficiency is a hereditary disease which increases the risk of venous thrombosis.

In some embodiments of the invention, the results obtained by the methods of the invention are compared to results obtained from a control blood sample. In some embodiments, the invention thus encompasses a method of testing for an antiphospholipid antibody in a patient, comprising adding heparin to a blood sample from said patient and determining the activity of heparin in said blood sample, and then comparing the activity of heparin in said blood sample to the activity of heparin in a control blood sample. In these embodiments of the invention, heparin resistance may then be determined, i.e. the ratio of the activity of heparin in the blood sample from a patient to the activity of heparin in a control blood sample.

The control blood sample is typically obtained from a healthy patient, or one who does not have antiphospholipid antibodies. The comparison of the activity of heparin in a blood sample from a patient with the activity of heparin in a control blood sample allows the determination of whether or not a patient has one or more antiphospholipid antibodies.

The present invention therefore also extends to a method of diagnosis of the presence of an antiphospholipid antibody or antiphospholipid antibodies. In one embodiment, lower activity of heparin in the blood sample obtained from the patient compared to the activity of heparin in the control blood sample is indicative of one or more antiphospholipid antibodies. In a second aspect, the present invention provides a method of testing for an antiphospholipid antibody in a patient, comprising adding heparin to a blood sample from said patient and determining the level of heparin in said blood sample.

In some embodiments of this aspect of the invention, for example when an anti-Xa assay is used to determine heparin levels, the heparin can be UFH or a LMWH, and is typically a LMWH. The LMWH can be any suitable LMWH, such as enoxaparin.

LMWHs are typically used in the methods of the invention at a concentration of 0.6 international units/millilitre (iu/ml) to 1.0 iu/ml, for example at a concentration of 0.6 iu/ml, 0.8 iu/ml, 0.9 iu/ml or 1.0 iu/ml.

In some embodiments of this aspect of the invention, for example when the anti-Xa assay is used to determine heparin levels, the blood sample is typically a plasma sample.

The level of heparin in the blood sample can be measured using any suitable means. One example of a test for measuring the level of heparin is the anti-Xa assay.

The anti-Xa assay is a chromogenic assay for the quantitative determination of UFH and LMWH anti-Xa activity in human plasma. The anti-Xa assay is a based on the ability of heparin to inhibit the activity of activated factor X (Xa) in the reagent. The reagent includes excess antithrombin, making the heparin in the sample the rate- limiting reagent for Xa inhibition. Factor Xa, for example bovine Factor Xa, is added in excess and heparin in the patient sample inhibits the enzymatic conversion of a Xa- specific chromogenic substrate to coloured product by factor Xa. Residual Factor Xa is then quantified with a synthetic chromogenic substrate. Standard curves are created using multiple concentrations of UFH and LMWH and are used to calculate concentration in the patient plasma. The colour generated is inversely proportional to the heparin level in the sample. In the present invention, the anti-Xa assay can be carried out using any suitable method known in the art, typically using commercially available products and reagents. For example, the anti-Xa assay can be carried out using the chromogenic substrate S2765 (Heparin HemosIL chromogenic assay (Instrumentation Laboratory (UK) Ltd, Warrington)) and/or using the ACL 300R analyzer (Instrumentation Laboratory (UK) Ltd, Warrington).

In some embodiments of this aspect of the invention, the invention encompasses a method of testing for an antiphospholipid antibody in a patient, comprising adding heparin to a blood sample from said patient and determining the level of heparin in said blood sample, and then comparing the level of heparin in said blood sample to the level of heparin in a control blood sample. In these embodiments of the invention, heparin resistance may then be determined, i.e. the ratio of the level of heparin in the blood sample from a patient to the level of heparin in a control blood sample.

The comparison of the level of heparin in the blood sample from a patient with the level of heparin in a control blood sample allows the determination of whether or not a patient has one or more antiphospholipid antibodies.

In an embodiment of this aspect of the invention, a lower level of heparin in the blood sample obtained from the patient compared to the level of heparin in the control blood sample is indicative of antiphospholipid antibodies.

The finding that when heparin is added to LA blood it produces less measured anticoagulant activity compared to blood from healthy controls means that the present invention can also be used to predict the response of a patient to anticoagulation treatment with heparin.

In this aspect of the invention, the method of the invention is used to determine whether or not a patient is responsive to anticoagulation treatment with heparin.

Typically, the activity of heparin in a blood sample from a patient is compared to the activity of heparin in a control blood sample, or the level of heparin in a blood sample from a patient is compared to the level of heparin in a control blood sample.

In one embodiment of this aspect of the invention, lower activity of heparin in the blood sample obtained from the patient compared to the activity of heparin in the control blood sample is indicative of a reduced response to heparin. In another embodiment, a lower level of heparin in the blood sample obtained from the patient compared to the level of heparin in the control blood sample is indicative of a reduced response to heparin.

In patients with a reduced response to heparin, it may be necessary to closely monitor heparin administration, for example during cardiopulmonary bypass, and/or to increase the amount of heparin administered to such patients during cardiac surgery and/or to effect anticoagulation by means of an anticoagulant other than heparin.

In one embodiment, the present invention provides a method of testing for an antiphospholipid antibody in a patient, comprising the following steps:

1. A sample of fresh venous blood, which has typically been withdrawn from a vein of a patient using flawless venepuncture, is placed into a machine for reading the

ACT and a baseline ACT reading is obtained, typically in seconds.

2. Just prior to venesection, unfractionated heparin is diluted with a suitable diluent, for example distilled water, to give a desired final concentration of unfractionated heparin, for example a concentration of 100 iu/ml. 3. Shortly after venesection, an aliquot of whole blood is added to a tube containing heparin to give a desired final concentration of unfractionated heparin. Optionally, a series of samples is produced with different concentrations of heparin in the samples, for example concentrations of 1 iu/ml, 3 iu/ml and 5 iu/ml. 4. The sample of heparinised blood is placed into a machine for reading the ACT and the ACT is recorded, typically in seconds. If different concentrations of heparin are being used, the ACT is recorded for each heparin concentration. 5. Optionally, steps 1 to 4 are repeated with a sample of blood from a healthy control, and the ACT of the blood sample from the control and the blood sample from the patient are then compared.

In another embodiment, the present invention provides a method of testing for an antiphospholipid antibody in a patient, comprising the following steps:

1. Immediately after venesection, a sample of whole blood which has typically been withdrawn from a vein of a patient using flawless venepuncture, is placed into a tube containing trisodium citrate.

2. A LMWH, such as enoxaparin, is added from to attain a desired level of anti- Xa activity. Optionally, a series of samples is produced with different levels of anti- Xa activity in the samples, for example of 0.2 iu/ml, 1 iu/ml, and 2 iu/ml.

2. The sample or samples containing LMWH is/are centrifuged to obtain plasma. For example, the sample(s) can be centrifuged at 3000rpm, at room temperature for

20 minutes.

3. Optionally, the resulting plasma is frozen, for example stored at -7O⁰C, and the sample(s) later defrosted.

4. The sample(s) is/are analysed for anti-factor Xa activity using a chromogenic substrate and a suitable analyzer, for example using the chromogenic substrate S2765 and/or the ACL 300R analyzer.

5. Optionally, steps 1 to 4. are repeated with a sample of blood from a healthy control and the anti-factor Xa activity in the blood sample from the control and the blood sample from the patient are compared.

Preferred features for the second aspect of the invention are as for the first aspect mutatis mutandis.

The invention will now be further described by way of reference to the following Example and Figure which are provided for the purposes of illustration only and are not to be construed as limiting on the invention. Reference is made to a Figure, in which: Figure 1 shows median values of ACTs in controls and LA patients according to dosage of UFH added.

Example 1

It was noted that patients with LA undergoing cardiac surgery, where large doses of heparin are required to prevent clotting in the cardiopulmonary bypass circuit, had unusual activated clotting time results (ACT). The effects of clinically utilised doses of heparin on heparin assays in blood samples taken from those with the LA versus healthy controls were therefore compared. In particular, the ACT, a near patient test for monitoring unfractionated heparin during cardiopulmonary bypass, and a standard chromogenic anti-Xa assay which is commonly used to monitor LMWH activity, were used.

Method

Ethics committee approval was obtained to conduct this observational study.

Subjects

Ten healthy controls (six men and four women), age range 23-49 years, and twenty adult patients with antiphospholipid syndrome, all female, and age range 24-45 years, gave verbal and written agreement to take part in the study. The LA patients fulfilled the criteria for antiphospholipid antibodies as they had previously tested positive for lupus anticoagulant by Dilute Russell Viper Venom testing (dRWT) or dilute activated partial thromboplastin time (aPTT) on two occasions at least 12 weeks apart and were recruited from outpatient clinics. The patients were retested at the time of the study and one was found to have a negative lupus anticoagulant at the time of the study and thus her results were excluded from analysis. Exclusion criteria were pregnancy and concurrent oral anticoagulant use. The use of aspirin was permitted.

Obstetric antiphospholipid syndrome was defined as the presence of persisting aPL associated with a history of three unexplained first trimester losses and/or intrauterine death and/or premature delivery before 34 weeks due to pre-eclampsia, placental abruption or intrauterine growth restriction. Patient demographics, including their primary diagnosis, antiphospholipid antibody profile and other concomitant diagnoses, are shown in Table 1.

Table 1 - Patient demographics

Key for Table 1:

LA- lupus anticoagulant

ACA- anticardiolipin antibody

Obs APS- Obstetric antiphospholipid syndrome

ITP-idiopathic thrombocytopenic purpura

SLE- systemic lupus erythematosus

C4 def- C4 deficiency

Assays

Thirty milliliters of fresh venous blood was withdrawn from the antecubital vein using a 21G cannula using flawless venepuncture. A 0.5 ml volume of blood was immediately pipetted into an "Actalyke" ACT tube (manufacturer Hemochron, activators; kaolin, celite and glass beads), gently agitated and a baseline ACT reading obtained in seconds (sees) using an Actalyke ® analyser, (Helena Laboratories GTA UK Ltd).

Just prior to venesection, unfractionated heparin (1000USP iu/ml) was diluted with distilled water to give a final concentration of 100 iu/ml. Within 2 minutes of venesection, aliquots of whole blood were added to tubes containing heparin so that final concentrations of unfractionated heparin (UH) in the samples analysed were 1 iu/ml, 3 iu/ml and 5 iu/ml to represent levels similar to those achieved during full heparinisation for cardiopulmonary bypass. From each heparin concentration 0.5ml volumes of blood were pipetted into ACT Actalyte tubes (multiple activators) using Actalyte ACT analyzer (GTA UK Ltd) machines and the ACT recorded. The intra and inter assay variability for the ACT was <10%.

Immediately after venesection, whole blood was pipetted into tubes containing 3.2% trisodium citrate and enoxaparin (a low molecular weight heparin) added from an initial concentration of 4000 iu/ml to attain levels of anti-Xa activity of 0.2 iu/ml, 1 iu/ml, and 2 iu/ml, which are similar to those used in the thromboprophylaxis and treatment. Each sample containing LMWH was centrifuged at 3000rpm, at room temperature for 20 minutes. The resulting plasma was stored at -70 ⁰C. Samples were later defrosted and analysed together for anti factor Xa activity using chromogenic substrate: S2765 (Heparin HemosIL chromogenic assay (Instrumentation Laboratory (UK) Ltd)). The assay was performed using the ACL 300R analyzer (Instrumentation Laboratory (UK) Ltd). The intra assay variability was 2.68, and the inter assay variability was 3.09.

In this assay, bovine Factor Xa is added in excess and neutralised by heparin- antithrombin complex. Residual Factor Xa is then quantified with a synthetic chromogenic substrate (S2765). Thus the colour generated is inversely proportional to the heparin level in the sample. As this was an ex vivo study, the results were expressed as optical densities as opposed to the anti-Xa activity. Statistical analysis was performed using XLSTAT (V2007.6, Addinsoft) Software. The data was first assessed for normality and found to be non-parametric. Thus comparisons were performed using the Mann Whitney u test (2-tailed) was used. A p value of p<0.05 was taken as statistically significant.

Results

Unfractionated heparin and ACT Table 2 shows the median and ranges of ACT values in seconds according to heparin dosages and Figure 1 shows the results graphically. The baseline ACTs in the LA group were not prolonged and did not differ significantly in comparison to the control group (Figure 1). However, as the unfractionated heparin concentration increased, the ACT' s were increasingly shorter in the LA group compared to the control group, reaching a statistically significant level at 3 iu/ml and 5 iu/ml of heparin.

Table 2 - Values of ACTs in controls and patients with LA according to the dosage of unfractionated heparin

LMWH and Optical densities

The LA plasma produced statistically significant higher optical densities in the chromogenic assay compared to controls at all points. The results are summarized in Table 3. Table 3 - Optical Density (OD) readings using a synthetic chromogenic substrate S2765 to measure colour generated in spiked LMW heparin samples of patients and controls

Discussion

These results show that when heparin is added to LA blood, it produces less measured anticoagulant activity compared to blood from healthy controls, when using the ACT and a chromogenic anti-Xa assay. .

The presence of lupus anticoagulant might be expected to prolong phospholipid dependent tests of anticoagulation including ACT. In this study, the baseline ACTs in the LA group were not prolonged and did not differ significantly in comparison to the control group. However, as the unfractionated heparin concentration increased, the ACTs were increasingly shorter in the LA group compared to the control group, reaching a statistically significant level at levels over 3 iu/ml. The accepted safe level of heparinisation for cardiopulmonary bypass is 3.0 iu/ml. At this concentration of heparin, the median ACT in the LA group was 337 sees compared to a control median of 421 sees. The threshold in common use for ACT considered adequate for commencement of bypass is 480 sees so both groups fell below this target. At the higher heparin level of 5 iu/ml the ACTs of both groups had surpassed 480 sees but there was a statistically significant difference in the two groups.

The optical density results were studied using a standard anti-Xa chromogenic assay and the same effect was found as shown by the ACT: statistically significant underreporting of anti-Xa activity in clinically utilised doses, since the levels of LMWH currently recommended for therapeutic anticoagulation are 0.6-1.0 iu/ml. The reduced anti-Xa levels seen in LA patients was consistent and increased gradually with larger doses.

The comparative effects of heparin on patients with LA compared to non LA individuals has not been studied before, and the results provided herein show that when heparin is added to LA blood, it produces less measured anticoagulant activity compared to blood from healthy controls, when using the ACT and a chromogenic anti-Xa assay.

In conclusion, this study shows that patients with LA had significantly reduced levels of heparinisation when monitored by two conventional assays. The study indicates that monitoring of heparin may be unreliable in patients with a lupus anticoagulant, with time shorter than predicted.

Claims

1. A method of testing for an antiphospholipid antibody in a patient, comprising adding heparin to a blood sample from said patient and determining the activity of heparin in said blood sample.

2. A method according to claim 1, wherein the activity of heparin in said blood sample is determined by measuring the activated clotting time (ACT) of said blood sample, wherein the blood sample is a sample of whole blood and wherein the heparin is unfractionated heparin (UFH).

3. A method according to claim 1 or 2, wherein the heparin is used at a concentration of 1 iu/ml to 5 iu/ml.

4. A method according to claim 3, wherein the heparin is used at a concentration of 3 iu/ml to 5 iu/ml.

5. A method according to claim 1, wherein the activity of heparin in said blood ^■sample is determined by measuring the activated partial thromboplastin time (aPTT) of said blood sample, wherein the blood sample is a sample of plasma and wherein the heparin is unfractionated heparin (UFH).

6. A method of testing for an antiphospholipid antibody in a patient, comprising adding heparin to a blood sample from said patient and determining the level of heparin in said blood sample.

7. A method according to claim 6, wherein the level of heparin in said blood sample is determined by carrying out an anti-Xa assay, wherein the blood sample is a sample of plasma.

8. A method according to claim 7, wherein the heparin is a low molecular weight heparin (LMWH).

9. A method according to claim 8, wherein the LMWH is exonaparin.

10. A method according to claim 8 or 9, wherein the heparin is used at a concentration of 0.6 iu/ml to 1 iu/ml.

11. A method according to any one of the preceding claims, wherein the antiphospholipid antibody is the lupus anticoagulant (LA).